The present invention relates generally to helical scan tape drives and, more particularly, to reading data of magnetic tape that is unreadable due to curved or displaced data tracks in a helical scan tape drive.
The helical systems of recording digital data on magnetic tape include a fixed head system and a helical scan system. The fixed head system forms recording tracks in the longitudinal direction on the magnetic tape by means of fixed heads. The helical scan system forms recording tracks at an angle with respect to the edge of the magnetic tape by means of rotary heads (i.e., helical heads). The helical scan system is useful for high density recording.
When data is recorded onto the magnetic tape, the data is not written verbatim because of the importance of data integrity. Instead, additional characters are calculated and added to the data as it is written. This process, known as error correction coding (ECC), allows the original data to be reconstructed at a later date even if a small number of data bits become damaged and unreadable.
When the helical system is instructed to read a certain file, the system positions the magnetic tape at the beginning of the file of interest and begins reading the data. The ECC is used to check the integrity of the data and correct any errors that are short in duration. The system stops reading the data and repositions to the beginning of the error region when the error is too long in duration for the ECC to correct. The system then attempts to reread the region of tape where the error occurred. Most read errors are caused by debris contaminating the read heads. The debris is sometimes dislodged during the reread attempt which makes the data readable. The reread operation is then attempted again if the data remains unreadable. The system quits after a certain number of retries and indicates that the data is unrecoverable.
Sometimes all or a large fraction of the data becomes unreadable for a long duration because the recorded helical data tracks are curved or are displaced from their specified locations. Curved tracks occur when the edge of the tape is damaged, the tape is subjected to improper tension, the tape path is misaligned, the tape is read by different head systems that have offsets, or debris contaminates the guides in the tape heads. Most practical ECC are only able to correct short and intermittent data errors. A long error, however, is generally not correctable by the ECC.
Accordingly, it is a general object of the present invention to provide a method and system for reading data of magnetic tape that is unreadable due to curved or displaced helical data tracks in a helical scan tape drive.
It is another object of the present invention to provide a method and system for reading data that is normally unreadable due to curved or displaced helical data tracks by controlling the position of the helical read heads with respect to the helical tracks as a function of historical position data of typical curved and displaced helical tracks.
It is a further object of the present invention to provide a method and system for reading data of magnetic tape that is unreadable due to curved or displaced helical data tracks by selecting one of a plurality of curved and displaced helical track profiles stored in memory and then controlling the position of the helical read heads with respect to the helical tracks as a function of the selected helical track profile.
In carrying out the above objects and other objects, the present invention provides a helical scan tape drive including a read head for reading a helical data track on magnetic tape to generate a read back signal. A position sensor generates a position signal indicative of a position of the read head with respect to the helical data track. A plurality of curve offset signals are stored in memory. Each one of the plurality of curve offset signals represents a curve profile of a respective given curved helical data track. A controller positions the read head as a function of the position signal and one of the plurality of curve offset signals such that the read head is positioned properly with respect to the helical data track to provide a proper read back signal when the helical data track is curved.
In further carrying out the above objects and other objects, the present invention provides a method for reading a helical data track of a helical scan tape drive. The method includes reading a helical data track on magnetic tape with a read head to generate a read back signal. A position signal is then generated to indicate a position of the read head with respect to the helical data track. A plurality of curve offset signals is stored in memory. Each one of the plurality of curve offset signals represents a curve profile of a respective given curved helical data track. The position of the read head is then controlled as a function of the position signal and one of the plurality of curve offset signals such that the read head is positioned properly with respect to the helical data track to provide a proper read back signal when the helical data track is curved.
These and other features, aspects, and embodiments of the present invention will become better understood with regard to the following description, pending claims, and accompanying drawings.